The present invention is concerned with seismic surveys. More specifically it concerns a device for, and a method for towing a seismic tow, consisting of a gun array and seismic streamers.
In marine seismic surveys a ship is used which tows seismic sources and seismic streamers. The sources produce acoustic energy which is propagated in the geological formations below the seabed where the survey is undertaken. The sound pulse or shock wave generated by the sources is reflected as acoustic waves. The seismic streamer consists of hydrophones which detect the reflected acoustic waves, convert the signals to electrical pulses, and pass on this information to a data acquisition system on board the vessel. Further processing and analysis of the information give an indication of the structure in the geological formations in which the survey has been conducted.
A seismic source consists of a number of individual sound sources, e.g. airguns or waterguns. An arrangement of individual guns is usually called a gun array. Apart from the actual guns a gun array also includes attachment devices, air hoses, electrical wires and sensors. A gun array measures 10-30 meters in length.
A seismic streamer, often called a hydrophone cable, consists mainly of hydrophones for the detection of acoustic waves, electronic modules, electrical wires and sensors apart from the actual framework. Seismic streamers are divided into sections approximately 100 meters in length, and have a total length of 2,000-6,000 meters.
A seismic tow or a seismic towing system consists of seismic streamers and seismic sources. A common feature of these units is that they can be positioned astern of and to the side of the ship's line of travel. In addition they are submerged in the ship's line of travel. In addition they are submerged in the water, the gun arrays at a depth of 5-15 meters below the surface and the streamers at a depth of 5-40 meters.
None of the above figures are absolute, and can, of course, vary outside those limits given here.
Seismic towing systems are becoming progressively more complex, i.e. they are composed of more sources and streamers. Increasing demands are also being made on the efficiency of a towing system. The efficiency can usually be measured on the basis of the extent of coverage obtained by a seismic tow. The coverage will be dependent on the width of the tow. One of the factors to which particular importance is attached is the positioning of the seismic tow. The quality of the collected data is dependent on how accurately the towing system has been positioned. In this context the term positioned refers to how each unit in the towing system, sources and streamers is positioned in relation to the others in the longitudinal and width directions.
One of the existing methods for positioning of seismic equipment which can be mentioned, is the use of otter-like devices with one or more foils. The otter is connected with a float on the surface, and by means of foils will guide a tow out to the side. There are several variations of the otter device. One known method is to have an individual foil or wing which can be mounted directly on to the streamer which has to be controlled. This type of otter device is also dependent on connection to a float on the surface.
Another method is to use surface bodies or vessels which are provided in such a manner that the vessel is towed at an angle, thus achieving a lateral force. Examples of this type of vessel which can be envisaged are vessels which are equipped with a tilted keel, or tilted foils, thus enabling the surface vessel to move out to the side.
The common feature of these methods and the equipment for obtaining a certain width for the towing system is that they all have a connection with the surface. This makes the vessels particularly vulnerable to flotsam. In the event of a collision between a vessel and some flotsam the towing system or parts of it will often be damaged. In a collision with flotsam the vessel could change direction, thus causing large sections of the towing system to be damaged. The same thing will happen if it comes into contact with fishing tackle or other equipment which may be in the location of the tow.
Another problem is that surface vessels or floats cause a great deal of friction. Severe friction gives rise to two problems in particular; increased fuel expenses and increased difficulty in obtaining tow width. The same will apply to the actual connection consisting of wires, rope, chains or the like between the gun array and the surface vessel or float, and the connection between the cable and the surface vessel or the float.
The fact that the gun arrays and the streamers are connected with the surface will cause waves to be generated. This gives rise to unwanted noise and vibrations for the towed equipment. In some cases there will also be problems with maintaining the equipment at a constant depth, especially when the survey is being conducted in heavy seas. It is also a problem to keep the equipment on the correct course, with the equipment usually drifting in step with the waves.
It is obvious that the equipment is subject to a considerable degree of wear and tear and damage when it is exposed to those forces which are associated with a connection with the surface.